As a related-art synchronization method, a plurality of known modulated waves for synchronization, which use a training symbol within a preamble or a pilot signal, are transmitted, and the known synchronizing signals are used to maintain the synchronization. Further, when a phase fluctuation is significant, an influence of a phase fluctuation amount is reduced by a differential modulation or the like (see, for example, Patent Literature 1).
FIG. 14 is a circuit configuration diagram of a related-art optical communication system. On a transmitter side, error correction coding is performed for transmission information by a forward error correction (FEC) coding unit, and a modulated signal based on quadrature phase shift keying (QPSK) or the like is generated by an optical modulation unit, to thereby perform transmission. In this case, high-speed optical communication has such a characteristic that the phase fluctuation is large. Therefore, in general, modulation is performed by mapping information to a phase difference between symbols (units of modulated/demodulated signals) on a transmitter side, while the demodulation is performed by using differential detection, differential synchronous detection, or the like for detecting the phase difference between the received symbols on a receiver side.
For example, even when a phase fluctuates by θ during a long segment of L symbols, a fluctuation of θ/L on average suffices in a case of the differential modulation. This produces such a characteristic that stable demodulation performance can be maintained because of being rarely influenced by the phase fluctuation.
On the other hand, when the phase fluctuates linearly by θ during the long segment of L symbols in synchronous detection for performing demodulation while perfectly maintaining synchronization between symbols, assuming that the order of the symbols is expressed as i=0, 1, 2, . . . , and L−1, i-th symbol is subject to the fluctuation of a phase of i×θ/L. Therefore, an influence exerted in the phase fluctuation becomes large, and in turn the demodulation performance greatly deteriorates.
However, in regard to the demodulation performance (error ratio after demodulation) on additive Gaussian noise (AWGN) with the phase fluctuation ignored, it is known that the differential detection and the differential synchronous detection are subject to deterioration of approximately 3 dB and 1.4 dB, respectively, with a BER of equal to or smaller than 1.E−5 points after decoding or the like compared with the synchronous detection.
Next, a description is made of a general phase compensation unit. A circuit illustrated in FIG. 14 is a block diagram of a phase compensation method based on a fourth-power method. Assuming that a linear distortion and an inter-channel nonlinear distortion have been compensated, a k-th symbol in a case where a QPSK demodulator is assumed can be expressed as follows:exp(jφd,k+jφk)+nk.
Here, φd,k represents a phase of a k-th piece of data, and takes any one value of ±π/4 or ±3π/4. Further, φk represents a phase shift that occurs due to phase noise caused by a laser or a nonlinear phase shift.
Next, with reference to FIG. 14, a description is made of a phase compensation unit 100.
(1) First, the received symbol is raised to the fourth power (corresponding to reference numeral 101 in FIG. 14). Here, φd,k is any one value of ±π/4 or ±3π/4, and hence the fourth power of exp(jφd,k) is always −1. As a result, a modulated data component can be removed by the fourth power calculation.
(2) Next, Gaussian noise nk can be removed by averaging (corresponding to reference numeral 102 in FIG. 14).
(3) Further, the fourth power of exp(jφd,k) is always exp(j4φd,k)=−1, and hence inverse compensation (corresponding to reference numeral 103 in FIG. 14) is performed, to extract only exp(j4φk) in principle.
(4) Finally, with respect to exp(j4φk), an angle is extracted by a function of angle Arg( ) and divided by 4, to thereby obtain an estimated phase shift value:{circumflex over (φ)}k  [Math. 1](corresponding to reference numeral 104 in FIG. 14). The value is converted into:exp(−{circumflex over (φ)}k)  [Math. 2](corresponding to reference numeral 105 in FIG. 14), and is multiplied by the received symbol (corresponding to reference numeral 106 in FIG. 14), to thereby remove a phase shift component.
With such operations as described above, a phase compensation circuit operates so as to allow correction based on the phase shift and reception at an appropriate signal point of the symbol. However, the fourth-power method raises a problem in that φk after averaging functions only when being within ±π/4, and when there is a fluctuation exceeding this range, operates so as to be locked in a position shifted by ±π/2 or π.
A consecutive phase shift caused in this manner when the phase is locked in a position shifted by ±π/2 or π (in other words, a phenomenon that a synchronization shift conspicuously occurs at ±π/2 and π in communications, and once synchronization is lost, a state of the synchronization shift continues) is expressed as “phase slip”. Due to this phenomenon, in the case of the synchronous detection, in regard to a portion that has caused the phase slip, a burst-like error occurs, which makes it difficult for an FEC decoding unit to correct the error. Therefore, the synchronous detection using this method is difficult for an optical communication device.
On the other hand, in cases of the differential detection and the differential synchronous detection, an error caused by the phase slip is only 1 bit (when the phase slip phenomenon gradually fluctuates over a plurality of symbols in actuality, a part of the number of bits included in the fluctuated segment). Accordingly, the differential detection and the differential synchronous detection are relatively robust against the phase slip phenomenon. For such reasons as described above, an optical communication system configuration based on the differential modulation is employed for a related-art method.